Organic solvent-based acrylic coatings have generally been used for coating of automobiles, household electric appliances, etc. because of their excellency in weatherability and appearance. Recently, however, it has become a task of the coating industry from the standpoint of environmental protection to allow organic solvent-based acrylic coatings to have a high solid content.
The high solid content can be achieved by means of decreasing the molecular weight of a resin used in coatings. For example, Journal of Coating Technology Vol. 59, No. 749, Jun. 1987 proposes using in high solid coatings a polymer of 650-3,100 in number-average molecular weight obtained by copolymerizing an alkyl acrylate, styrene, acrylic acid, etc. in the presence of 3-mercaptopropionic acid.
Other methods for achieving the high solid content include using a branched-chain polymer as a resin for coating, and there has been proposed the following branched-chain polymer, for example.
That is, according to Japanese Patent Application Kokai No. 4-103610, a branched-chain polymer obtained by copolymerizing a radical-polymerizable macromolecular monomer containing, as the skeleton, a polymer obtained by radical polymerization of an alkyl (meth)acrylate, etc. in the presence of a mercaptan, with another monomer, gives a low-viscosity solution even at a high solid content and is therefore suitable as a resin for high-solid coating.
A representative process for synthesis of the above radical-polymerizable macromolecular monomer is described below in detail.
That is, a radical-polymerizable monomer is polymerized in the presence of a mercaptan having a thiol group and a carboxyl group such as thioglycollic acid or mercaptopropionic acid to obtain a polymer of 1,000-20,000 in number-average molecular weight having a carboxyl group at one end of the molecule. Then, the polymer is reacted with a monomer having an addition-reactive group such as glycidyl methacrylate or the like to obtain a macromolecular monomer having a methacryloyl group at one end of the molecule.
The polymer or macromolecular monomer obtained by radical polymerization using a mercaptan as a chain transfer agent, however, has had a problem of inferior weatherability, because the polymer or macromolecular monomer contains a thioether bond which is easily deteriorated by ultraviolet rays. Branched-chain polymers containing said macromolecular monomer as a component have had the same problem.